finished ch 7.4

Packages.md

@@ -8,4 +8,27 @@ packages can have as many binary crates as you want but only at most one library
 
 packages must have at least one crate
 
-packages are defined by the toml file that comes with creating a new crate
+packages are defined by the toml file that comes with creating a new crate
+
+# External Packages
+
+These crates are normally found on [crates.io](https://crates.io/) 
+in the guessing game the external crate was first defined in the cargo.toml as
+```toml
+rand = "0.8.5"
+```
+
+it was then brough into scope by
+```rust
+use rand::Rng;
+// and then used
+rand::thread_rng().gen_range(0..=100);
+```
+
+pulling other modules is often the same process
+
+the std library is an external package but it doesn't need to be specified in the toml file
+
+but it does need to be brought into scope with the ``use`` keyword
+
+the std::other_part is an absolute path starting with std for the standard library crate